Kinetics and mechanism of oxidation of benzoic acid hydrazide by bromate catalyzed by octamolybdomanganate(II)

Oxidation of benzoic acid hydrazide by bromate in the presence of octamolybdomanganate(II), [Mn-II Mo8O27](4-), was studied in hydrochloric acid medium. The mechanism of the reaction involves oxidation of the catalyst to [Mn-IV Mo8O27](2-) by bromate which then forms a complex with the unoxidized catalyst. Both the complex and [MnIV Mo8O27](2-) react with the substrate in rate-determining steps to generate an intermediate acyl diimide, RCONNH. The reaction of water with the diimide then leads to the formation of benzoic acid and nitrogen as products through an NH-NH intermediate. There was no formation of free radical, indicating the involvement of only two-electron transfer steps in the mechanism. The order of more than unity in catalyst concentration is due to the formation of complex between the catalyst and the oxidized form of the catalyst. A rate law explaining all the kinetic results has been derived and verified. The effects of ionic strength and solvent polarity have also been studied, and the thermodynamic parameters were determined. A less solvated transition state as a result of interaction between the complex and oxidized form of the catalyst satisfactorily explains all the effects observed.